Ongoing studies of clinical and immunological parameters of hay fever in children and adults were designed to provide information on the mechanism by which injection therapy results in clinical improvement. Symptom scores are obtained with diary cards, and then correlated with serum IgE antibody levels (measured by passive leukocyte sensitization and the radioallergosorbant test). A long term study of hay fever in nonimmunized children is being initiated. We are characterizing a receptor for IgE on the HC subline of the Furth mouse mastocytoma. The receptor is demonstrated by determining the reduction in titer of IgE antibody measured by passive cutaneous anaphylaxis after absorption with intact HC cells. We have developed a more sensitive assay for histidine decarboxylase and will use it to determine the effect of histamine release on the activity of this enzyme in mast cells and basophils. We have initiated a study of allergens in animal danders, in particular, those from rodent skin. Sensitivity to allergenic proteins is determined with the in vitro leukocyte histamine release system. Skin extracts will be fractionated by chromatographic and electrophoretic methods to prepare purified allergenic proteins for further biological and chemical characterization.